test: make p2p_handshake robust against timeoffset warnings #29704

The new p2p_handshake test requires that limited nodes are not peered with when the node's system time exceeds ~ 24h of the node's chaintip timestamp, as per PeerManagerImpl::GetDesirableServiceFlags.

By patching this test to modify the timestamp of the chaintip as opposed to mocking the node's system time, we make it resilient to future commits where the node raises a warning if it detects its system time is too much out of sync with its outbound peers.

Resolves a silent merge conflict in #29623, that is changing the warning behaviour when significant time differences with outbound peers are detected, failing the test as it's currently in master.

Considerations/alternatives I've thought of:

• could add self.setup_clean_chain = True to self.set_test_params(), to avoid creating a new tip with a (much) older date, but it doesn't seem to matter?
• could avoid using setmocktime altogether and instead use create_block instead, but that seems like it'll be a lot more verbose and I don't think it's worth it?

Big thanks to theStack for his time in discussing this with me offline.

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lgtm ACK 032a59748295859845b2a9181ceb1c4ae70bae5c

A third alternative might be to restart the node after each test to drop the state, but that seems fragile.

Please don't @ (ping) in the OP, which ends up in the merge commit messeage

ACK 032a59748295859845b2a9181ceb1c4ae70bae5c

crACK 032a59748295859845b2a9181ceb1c4ae70bae5c

Code Review ACK 032a59748295859845b2a9181ceb1c4ae70bae5c